1. Field of the Invention
The present invention relates to electrophotographic transfer paper which is utilized for image formation through an electrophotographic process applied to a color copying machine, a color printer, or the like.
2. Description of the Related Art
Conventionally, examples of a method of forming a color image through an electrophotographic process include a color image forming method in which a photoconductor is irradiated with a color separation light to form an electrostatic latent image in each color; the electrostatic latent image of each color is sequentially developed with color toners such as Y (yellow), M (magenta), and C (cyan) to form a color toner image in each color; each color toner image is transferred by superimposing the color toner image on a transfer body each time the color toner image is formed; and these color toner images are heated, melted, and fixed to the transfer body.
Another example of a color image forming method is a color image forming method in which each color toner image is formed by superimposing the color toner image not on the transfer body but on the photoconductor; the superimposed color toner images are collectively transferred onto the transfer; and these color toner images are heated, melted, and fixed to the transfer body. Still another example of a color image forming method is a method in which the toner image is indirectly transferred from the photoconductor to the transfer body by using an intermediate transfer body such as a belt; specifically, this is a method in which the color toner image formed on the photoconductor in each color is superimposed on the intermediate transfer body, the superimposed color toner images are collectively transferred onto the transfer body, and these color toner images are heated, melted, and fixed to the transfer body.
The color toner is formed by mutually solving various dyes in a binder resin or by dispersing various pigments in the binder resin as coloring agents. Particle size of the color toner ranges from several micrometers to tens of micrometers. Paper substrates such as plain paper, general printing paper, and coated paper are used as acceptors for the color toner. A plurality of layers of the color images superimposed on the paper substrate are heated, melted, and fixed to form the color image (see Japanese Patent Laid-Open Application (JP-A) No. 63-92965). Because depressions and projections ranging from 10 to 100 μm are formed on a surface of the color image formed in the above-described way, the depressions and projections on the surface of the color image generate unevenness in gloss of the image.
Additionally, in order to solve the above-described problem, various methods in which a transparent thermoplastic resin layer is provided as an acceptor layer on the substrate and the toner image is embedded into the transparent thermoplastic resin layer with a heated roller fixing device are well known.
Examples of the above-described method include a method in which the toner image is transferred onto the surface of an image transfer sheet having a transparent resin layer comprising a crosslinked resin which has a glass transition temperature ranging from 40 to 70° C., and is soluble in tetrahydrofuran, and the toner image is embedded into the transparent resin layer with a belt-shaped fixing device (see JP-A No. 5-127413). Another example of the above-described method is a method in which the toner image is transferred onto the surface of the image transfer sheet; which surface is coated with the thermoplastic resin; and the toner image is embedded into the transparent resin layer with a belt-shaped fixing device (see JP-A Nos. 5-216322 and 6-11982).
Examples of an image forming method for obtaining a gloss-matched image with even gloss include a method in which the average molecular weight (Mwa) of the transparent resin provided on the surface of the transfer sheet and the average molecular weight (Mwb) of a binding resin of the color toner have a relationship shown by Mwa−Mwb≧10000 and the image is formed with the electrophotographic transfer paper in which a melting tilt angle of the color toner to the binding resin adjusted to not more than 40 degrees at a toner-fixing temperature of the transparent resin (see JP-A No. 10-221877).
Another example of an image forming method is a method in which the image is formed with an electrophotographic transfer paper with a layer of transparent resin whose number average molecular weight (Mn) ranges from 5000 to 20000 and whose glass transition temperature ranges from 30 to 85° C. (see JP-A No. 11-160905). Still another example of the image forming method is a method in which a plasticizer is mixed in the thermoplastic resin layer and a binder or a solid component for forming a layer is softened during the fixing to embed the toner into the thermoplastic resin layer (see JP-A No. 2000-275891).
In the conventional art indicated in the above-described references, the color toner image is heated and melted to embed the color toner into the transparent resin layer on the surface of the electrophotographic transfer paper by pressing the heated roller against the color toner image when fixing the color toner image onto the transfer body.
When utilizing these methods to form an image, in a low image-density area, the toner image is embedded into the thermoplastic resin layer, and the smoothness of the whole image forming surface is enhanced and the gloss improved. However, in a high image density area, the toner image cannot be completely embedded into the thermoplastic resin layer, and a level gradation occurs whereby the image portion of the image forming surface is raised higher than the non-image portion (hereinafter, also referred to as an “image step”). The image step gives a visual impression that something is wrong with the image and, further, the existence of the image step reduces the gloss such that an image is provided with a large difference in gloss over the whole image forming surface.
As methods for eliminating the image height of the toner or the gloss difference of the whole image, methods of eliminating the impaired visual impression and the difference in gloss between the image and non-image portions caused by the image step by providing a porous coating layer (thermoplastic foamed resin layer) on a support body and embedding the toner in pores on the surface of the porous coating layer, have been proposed (see JP-A Nos. 9-171266, 11-282192, and 2000-292961).
When an image is formed using such conventional art, the toner is easily embedded into the pores on the surface of the porous coating layer during the fixing, so that the image step can be reduced and the visual impression that something is wrong and the gloss difference caused by the image step can be suppressed, when compared with paper on which a thermoplastic resin layer is provided.
Further, in the above-described art, in addition to the improvement of the toner embedding properties, the gloss difference is suppressed by decreasing the gloss itself in the image portion and the non-image portion. Therefore, the gloss of the whole image forming surface tends to be slightly decreased, and sometimes a paper on which a porous coating layer is provided is not suitable for production of a document in which a glossier finish is required.
A laminated type porous sheet in which at least two porous coating layers are provided on a support body has also been proposed as an another art which provides a porous coating layer on a support layer (see JP-A No. 11-10762). However, in the art, since at least two porous coating layers are provided on the support body, the heat capacity of the whole porous coating layer is remarkably increased. Therefore, depending on the toner used or the fixing conditions, the toner and the porous coating layer are not sufficiently melted during the fixing and sometimes an image with a sufficiently glossy finish is not obtained.
Namely, on paper on which a porous coating layer is provided on the surface, sometimes the gloss finish is insufficient even if the gloss difference of the whole image forming surface can be suppressed.
In view of the above-described problems in the conventional art, the present invention has been devised in order to provide an electrophotographic transfer paper in which the toner is embedded well in the thermoplastic foamed resin layer suring fixing, the gloss difference over the whole image forming surface is eliminated, and the image has a glossy finish, and to provide an image forming method using the electrophotographic transfer paper.